Method of forming steel sheet, equipment for forming the same, and formed product

ABSTRACT

A formed product of a steel sheet is provided. The formed product includes a surface portion adjacent to an exterior surface and a central portion surrounded by the surface portion. A first phase has a maximum volume fraction in the surface portion, and a second phase has a minimum volume fraction in the surface portion. The second phase has a maximum volume fraction in the central portion, and the first phase has a minimum volume fraction in the central portion.

BACKGROUND

1. Field

Embodiments of the inventive concepts relate to a method of forming a steel sheet, equipment for forming the same, and a formed product. More particularly, embodiments of the inventive concepts relate to a method of forming a steel sheet including a surface portion and a central portion having a different structure from the surface portion, equipment for forming the same, and a formed product.

2. Description of the Related Art

High-strength steel sheets may be used as future automotive materials (e.g., lightweight materials of cars and new-concept automotive part materials) and may be used for high-strength reinforcing bars and materials resistant to repeated fatigue. The high-strength steel sheets may also be used as high-strength and high vibration damping materials for shipbuilding because of their excellent low-temperature toughness and high tensile strength. In addition, the high-strength steel sheets may also be used as materials for welding and materials for pipes. Thus, application fields and markets for the high-strength steel sheets are increasing.

Generally, an elongation of the steel sheet decreases as the strength of the steel sheet increases, and thus workability of the steel sheet is deteriorated. Steel makers are conducting researches to maximize the advantages of the high-strength steel sheets by securing both strength and workability characteristics which conflict with each other.

To resolve this problem, Korean Patent Publication No. 2000-0043784 (Korean patent application No. 10-1998-0060205, Applicant: POSCO) discloses a high-strength hot-rolled steel sheet composed of carbon of 0.06 wt % to 0.1 wt %, silicon of 0.3 wt % or less, manganese of 1.4 wt % to 2.0 wt %, phosphorus of 0.02 wt % or less, sulfur of 0.005 wt % or less, aluminum of 0.01 wt % to 0.05 wt %, titanium of 0.05 wt % to 0.15 wt %, niobium of 0.02 wt % to 0.04 wt %, nitrogen of 50 ppm or less, a residual iron (Fe), and other inevitable impurities and a method of manufacturing the same. This high-strength hot-rolled steel sheet has a tensile strength of about 70 kg/mm².

Korean Patent Publication No. 2001-0060647 (Korean patent application No. 10-1999-0063053, Applicant: POSCO) discloses a hot-rolled steel sheet composed of carbon of 0.06 wt % to 0.10 wt %, silicon of 0.5 wt % to 1.0 wt %, manganese of 1.5 wt % to 2.0 wt %, phosphorus of 0.02 wt % or less, sulfur of 0.0005 wt % or less, aluminum of 0.010 wt % to 0.050 wt %, titanium of 0.050 wt % to 0.10 wt %, niobium of 0.020 wt % to 0.040 wt %, nitrogen of 60 ppm or less, a residual iron (Fe), and other inevitable impurities and a method of manufacturing the same. This hot-rolled steel sheet has excellent workability and a tensile strength of about 780 MPa.

SUMMARY

Embodiments of the inventive concepts may provide a method of forming a steel sheet to manufacture a formed product having high strength, equipment for forming the same, and the formed product.

Embodiments of the inventive concepts may also provide a method of forming a steel sheet to manufacture a formed product having high elongation, equipment for forming the same, and the formed product.

Embodiments of the inventive concepts may further provide a method of forming a steel sheet capable of easily being applied to general processes, and equipment for forming the same.

In an aspect, a method of forming a steel sheet may include preparing a steel sheet including a surface portion and a central portion, thermally treating the steel sheet, performing a first phase transformation process cooling a surface of the thermally treated steel sheet to phase-transform a structure of the surface portion of the thermally treated steel sheet, forming the steel sheet after the performing of the first phase transformation process, and performing a second phase transformation process cooling the formed steel sheet to phase-transform a structure of the central portion of the formed steel sheet.

In some embodiments, a phase of the structure of the central portion of the steel sheet may be maintained during the performing of the first phase transformation process.

In some embodiments, a phase of the structure of the surface portion of the steel sheet may be maintained during the forming of the steel sheet and the performing of the second phase transformation process.

In some embodiments, the thermally treated steel sheet may have an austenite structure. The structure of the surface portion of the thermally treated steel sheet may be phase-transformed to a ferrite structure by the first phase transformation process, and the austenite structure of the central portion of the thermally treated steel sheet may be maintained during the performing of the first phase transformation process. The central portion of the formed steel sheet may have a structure having a higher strength than the ferrite structure by the second phase transformation process, and the ferrite structure of the surface portion of the formed steel sheet may be maintained during the performing of the second phase transformation process.

In some embodiments, an elongation of the surface portion phase-transformed by the first phase transformation process may be higher than an elongation of the central portion phase-transformed by the second phase transformation process, and a strength of the central portion phase-transformed by the second phase transformation process may be higher than a strength of the surface portion phase-transformed by the first phase transformation process.

In some embodiments, the performing of the first phase transformation process may include supplying compressed air to the surface portion of the steel sheet.

In some embodiments, the performing of the first phase transformation process may include jetting a liquid temperature-reducing agent to the surface portion of the steel sheet.

In an aspect, a formed product may include a surface portion adjacent to an exterior surface, and a central portion surrounded by the surface portion. A first phase may have a maximum volume fraction in the surface portion, and a second phase may have a minimum volume fraction in the surface portion. The second phase may have a maximum volume fraction in the central portion, and the first phase may have a minimum volume fraction in the central portion.

In some embodiments, a structure of the first phase may be a ferrite structure, and a structure of the second phase may have a higher strength than the ferrite structure.

In some embodiments, the surface portion may have only the first phase, and the central portion may have only the second phase.

In some embodiments, the volume fraction of the first phase may gradually decrease from the surface portion to the central portion, and the volume fraction of the second phase may gradually increase from the surface portion to the central portion. The volume fraction of the second phase may gradually decrease from the central portion to the surface portion, and the volume fraction of the first phase may gradually increase from the central portion to the surface portion.

In some embodiments, a structure of the first phase may have a higher elongation than a structure of the second phase, and the structure of the second phase may have a higher strength than the structure of the first phase.

In an aspect, equipment for forming a steel sheet may include a cutting part cutting a steel sheet, a heat supply part thermally treating the cut steel sheet, a surface cooling part selectively cooling a surface portion of the thermally treated steel sheet, and a forming/cooling part forming the steel sheet having the cooled surface portion and cooling a central portion of the formed steel sheet.

In some embodiments, the surface cooling part may include a first surface cooling part cooling a first surface of the steel sheet, and a second surface cooling part cooling a second surface of the steel sheet opposite to the first surface.

In some embodiments, the first surface cooling part and the second surface cooling part may supply a temperature-reducing agent to the first surface and the second surface in a direction opposite to a direction in which the steel sheet is transferred.

In some embodiments, temperature-reducing agent supply nozzles supplying the temperature-reducing agent may be disposed obliquely to the first and second surfaces of the steel sheet.

In some embodiments, the surface cooling part may selectively cool the surface portion of the thermally treated steel sheet except the central portion to phase-transform a structure of the surface portion, and the forming/cooling part may cool the formed steel sheet to phase-transform a structure of the central portion except the surface portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart illustrating a method of forming a steel sheet according to some embodiments of the inventive concepts.

FIG. 2 is a flow chart illustrating a method of forming a steel sheet according to other embodiments of the inventive concepts.

FIG. 3 is a perspective view illustrating a formed product according to some embodiments of the inventive concepts.

FIG. 4 is a cross-sectional view taken along a line A-A′ of FIG. 3 to illustrate a hot-rolled steel sheet according to some embodiments of the inventive concepts.

FIG. 5 illustrates volume fractions of phases of a cross section taken along the line A-A′ of FIG. 3.

FIG. 6 is a cross-sectional view taken along the line A-A′ of FIG. 3 to illustrate a formed product according to some embodiments of the inventive concepts.

FIG. 7 is a schematic view illustrating equipment for forming a steel sheet, according to some embodiments of the inventive concepts.

FIG. 8 is a perspective view illustrating a surface cooling part included in equipment for forming a steel sheet, according to some embodiments of the inventive concepts.

FIG. 9 is a schematic view illustrating equipment for forming a steel sheet, according to other embodiments of the inventive concepts.

FIG. 10 is a schematic view illustrating equipment for forming a steel sheet, according to still other embodiments of the inventive concepts.

FIG. 11 is a scanning electron microscopy (SEM) image illustrating micro-structures of a steel sheet according to some embodiments of the inventive concepts.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The inventive concepts will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the inventive concepts are shown. It should be noted, however, that the inventive concepts are not limited to the following exemplary embodiments, and may be implemented in various forms. Accordingly, the exemplary embodiments are provided only to disclose the inventive concepts and let those skilled in the art know the category of the inventive concepts.

It will be understood that when an element such as a layer, region or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements may be present. In addition, in the drawings, the thicknesses of layers and regions are exaggerated for clarity.

It will be also understood that although the terms first, second, third etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another element. Thus, a first element in some embodiments could be termed a second element in other embodiments without departing from the teachings of the present invention. Exemplary embodiments of aspects of the present inventive concepts explained and illustrated herein include their complementary counterparts. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used herein, the singular terms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “includes”, “including”, “have”, “has” and/or “having” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. Furthermore, it will be understood that when an element is referred to as being “connected” or “coupled” to another element, it may be directly connected or coupled to the other element or intervening elements may be present.

In addition, in explanation of the present invention, the descriptions to the elements and functions of related arts may be omitted if they obscure the subjects of the inventive concepts.

FIG. 1 is a flow chart illustrating a method of forming a steel sheet according to some embodiments of the inventive concepts.

Referring to FIG. 1, a steel sheet is prepared (S110). In some embodiments, the steel sheet may be a hot-rolled steel sheet. Alternatively, in certain embodiments, the steel sheet may be a cold-rolled steel sheet. However, embodiments of the inventive concepts are not limited to a kind of the steel sheet.

The steel sheet may be thermally treated (S120). A structure of the thermally treated steel sheet may be phase-transformed. For example, the steel sheet may have an austenite structure. In some embodiments, both a surface portion and a central portion of the steel sheet may have the austenite structure.

A surface of the thermally treated steel sheet may be cooled to perform a first phase transformation process that phase-transforms the structure of the surface portion of the steel sheet (S130).

In the first phase transformation process, the surface of the thermally treated steel sheet may be cooled to phase-transform the structure of the surface portion of the steel sheet. The surface portion of the steel sheet may be portions of the steel sheet which are adjacent to a first surface and a second surface of the steel sheet. The second surface is opposite to the first surface. The first surface and the second surface may be a top surface and a bottom surface of the steel sheet, respectively.

For example, when the thermally treated steel sheet has the austenite structure before performing the first phase transformation process, the surface portion of the steel sheet may have a ferrite structure by the first phase transformation process.

The phase of the structure of the central portion of the steel sheet may be maintained while the structure of the surface portion of the steel sheet is phase-transformed by the first phase transformation process. For example, even though the first phase transformation process is performed, the central portion of the steel sheet may have the austenite structure. In other words, the phase of the structure of the central portion of the steel sheet may not be transformed during the first phase transformation process. The central portion of the steel sheet may be a portion of the steel sheet, which is disposed between the first surface and the second surface of the steel sheet.

In some embodiments, compressed air may be provided to the first surface and the second surface of the steel sheet during the first phase transformation process. For example, air in the atmosphere may be compressed using a pump to prepare the compressed air. Alternatively, the compressed air may be prepared using a specific gas such as an inert gas.

In other embodiments, a liquid temperature-reducing agent may be jetted to the first surface and the second surface of the steel sheet during the first phase transformation process. For example, the liquid temperature-reducing agent may be water or liquid nitrogen.

The first and second surfaces of the steel sheet may be cooled to a temperature of A3 or less by the compressed air or liquid temperature-reducing agent provided to the first surface and the second surface, and thus the structure of the surface portion of the steel sheet may be phase-transformed. For example, the structure of the surface portion of the steel sheet may be phase-transformed from the austenite structure to the ferrite structure.

The compressed air or liquid temperature-reducing agent may be supplied to the first surface and the second surface of the steel sheet for a short time. Thus, the central portion of the steel sheet may not be cooled during the first phase transformation process such that the phase of the structure of the central portion may be maintained.

After the structure of the surface portion of the steel sheet is phase-transformed, a process of forming the steel sheet may be performed (S140). The process of forming the steel sheet may be performed by pressing the steel sheet of which the structure of the surface portion is phase-transformed. The steel sheet may be formed to manufacture a formed part.

The formed steel sheet may be cooled to perform a second phase transformation process that phase-transforms the structure of the central portion of the formed steel sheet (S150). The structure of the central portion of the formed steel sheet, which was not phase-transformed in the first phase transformation process as described above, may be phase-transformed by the second phase transformation process.

For example, when the central portion of the steel sheet, which was not phase-transformed in the first phase transformation process, has the austenite structure, the structure of the central portion of the steel sheet may be phase-transformed to a martensite or bainite structure by the second phase transformation process. Alternatively, the central portion of the steel sheet may have a high strength low alloy (HSLA) steel structure such as a dual phase (DP) steel structure, a transformation induced plasticity (TRIP) steel structure, or a twinning induced plasticity (TWIP) steel structure after the second phase transformation process.

The phase of the structure of the surface portion of the formed steel sheet may be maintained while the structure of the central portion of the formed steel sheet is phase-transformed by the second phase transformation process. For example, even though the second phase transformation process is performed, the surface portion of the steel sheet may have the ferrite structure. In other words, the phase of the structure of the surface portion of the steel sheet may not be transformed during the second phase transformation process.

An elongation of the structure, which is phase-transformed in the second phase transformation process, of the central portion of the steel sheet may be lower than an elongation of the structure, which is phase-transformed in the first phase transformation process, of the surface portion of the steel sheet. The strength of the structure, which is phase-transformed in the second phase transformation process, of the central portion of the steel sheet may be higher than the strength of the structure, which is phase-transformed in the first phase transformation process, of the surface portion of the steel sheet.

The steel sheet on which the first phase transformation process was performed may be water-cooled in the second phase transformation process. In some embodiments, the second phase transformation process may be performed using an apparatus different from an apparatus of performing the first phase transformation process. In some embodiments, the second phase transformation process and the forming process may be performed in the same apparatus. In other words, the second phase transformation process may be performed in the apparatus of forming the steel sheet immediately after the steel sheet is formed.

As described above, the surface portion and the central portion may have the structures different from each other. For example, the surface portion may have the ferrite structure, and the central portion may have the martensite or bainite structure. Meanwhile, a portion (e.g., a mixed portion) of the steel sheet, which is disposed between the surface portion and the central portion of the formed steel sheet, may have a dual phase structure.

The mixed portion of the steel sheet may be phase-transformed by the first phase transformation process and may be phase-transformed by the second phase transformation process. In more detail, a structure of a portion of the mixed portion may be phase-transformed by the first phase transformation process, and a structure of the rest portion of the mixed portion may be phase-transformed by the second phase transformation process.

While the structure of the portion of the mixed portion is phase-transformed by the first phase transformation process, the structure of the rest portion of the mixed portion may not be phase-transformed. While the structure of the rest portion of the mixed portion is phase-transformed by the second phase transformation process, the portion of the mixed portion previously phase-transformed in the first phase transformation process may not be phase-transformed. Thus, the mixed portion may include a first phase transformed in the first phase transformation process and a second phase transformed in the second phase transformation process.

A volume fraction of the first phase may be higher than a volume fraction of the second phase in regions of the mixed portion, which are adjacent to the first surface and the second surface. On the other hand, a volume fraction of the second phase may be higher than a volume fraction of the first phase in a region of the mixed portion, which is adjacent to the central portion.

For example, when the thermally treated steel sheet has the austenite structure before the first phase transformation process, the portion of the mixed portion of the steel sheet may be phase-transformed from the austenite structure to the ferrite structure by the first phase transformation process. The austenite structure of the rest portion of the mixed portion may be maintained while the portion of the mixed portion is phase-transformed to the ferrite structure. The rest portion of the mixed portion may be phase-transformed from the austenite structure to the martensite or bainite structure by the second phase transformation process. The ferrite structure of the portion of the mixed portion may be maintained while the rest portion of the mixed portion is phase-transformed to the martensite or bainite structure. Thus, the mixed portion may have the dual phase structure including the ferrite and martensite structures.

According to some embodiments of the inventive concepts, the surface portion of the steel sheet may have the structure (e.g., the ferrite structure) having a relatively high elongation by the first phase transformation process, and the central portion of the steel sheet may have the structure (e.g., the martensite or bainite structure) having a relatively high strength by the second phase transformation process. Thus, the formed steel sheet (e.g., the formed part) may have the high elongation and the high strength.

According to other embodiments, the first phase transformation process may be performed after the process of forming the steel sheet, unlike the method of forming the steel sheet described with reference to FIG. 1. This will be described with reference to FIG. 2.

FIG. 2 is a flow chart illustrating a method of forming a steel sheet according to other embodiments of the inventive concepts.

Referring to FIG. 2, a steel sheet may be prepared (S110) and the steel sheet may be thermally treated (S120), as described with reference to FIG. 1. A structure of the thermally treated steel sheet may be phase-transformed to have, for example, an austenite structure, as described with reference to FIG. 1. A process of forming the thermally treated steel sheet may be performed by the method described with reference to FIG. 1 (S130). A phase of the structure of the thermally treated steel sheet may be maintained in the forming process.

After the thermally treated steel sheet is formed, a surface of the formed steel sheet may be cooled according to the method described with reference to FIG. 1 to perform a first phase transformation process which phase-transforms a structure of a surface portion of the formed steel sheet (S140). Thereafter, the formed steel sheet on which the first phase transformation process was performed may be cooled to perform a second phase transformation process which phase-transforms a structure of a central portion of the formed steel sheet (S150).

In some embodiments, the first phase transformation process and the second phase transformation process may be sequentially performed in the same apparatus. Alternatively, the first phase transformation process and the second phase transformation process may be performed in apparatuses different from each other.

A formed product manufactured by the method of forming the steel sheet according to the aforementioned embodiments of the inventive concepts will be described hereinafter with reference to FIGS. 3 to 5.

FIG. 3 is a perspective view illustrating a formed product according to some embodiments of the inventive concepts, FIG. 4 is a cross-sectional view taken along a line A-A′ of FIG. 3 to illustrate a hot-rolled steel sheet according to some embodiments of the inventive concepts, and FIG. 5 illustrates volume fractions of phases of a cross section taken along the line A-A′ of FIG. 3.

Referring to FIGS. 3 to 5, a formed product 10 may include a first surface 10 a, a second surface 10 b, and a central portion. The first surface 10 a and the second surface 10 b may be opposite to each other. The central portion may be a portion of the formed product 10, which is disposed between the first surface 10 a and the second surface 10 b.

The formed product 10 may have a dual phase structure having a first phase and a second phase. The first phase of the formed product 10 may be generated in the first phase transformation process described with reference to FIGS. 1 and 2, and the second phase of the formed product 10 may be generated in the second phase transformation process described with reference to FIGS. 1 and 2.

The first phase may have the maximum volume fraction at the first surface 10 a and the second surface 10 b. The formed product 10 may have a first portion adjacent to the first surface 10 a and a second portion adjacent to the second surface 10 b. In some embodiments, the first and second portions of the formed product 10 may have only the first phase but may not have a structure of the second phase (hereinafter, referred to as ‘a second phase structure’). In other embodiments, the first and second portions may have a very small amount of the second phase structure. The volume fraction of the first phase may gradually decrease from the first surface 10 a to the central portion, and the volume fraction of the second phase may gradually increase from the first surface 10 a to the central portion. The first and second portions may be surface portions adjacent to exterior surfaces of the formed product 10, and the central portion may be an inner portion of the formed product 10, which is surrounded by the surface portions.

The second phase may have the maximum volume fraction in the central portion. In some embodiments, the central portion of the formed product 10 may have only the second phase but may not have a structure of the first phase (hereinafter, referred to as ‘a first phase structure’). In other embodiments, the central portion of the formed product 10 may have a very small amount of the first phase structure. The volume fraction of the second phase may gradually decrease from the central portion to the first surface 10 a and from the central portion to the second surface 10 b, and the volume fraction of the first phase may gradually increase from the central portion to the first surface 10 a and from the central portion to the second surface 10 b.

The first phase structure may have a higher elongation than the second phase structure. The second phase structure may have a higher strength than the first phase structure. For example, the first phase structure may be a ferrite structure, and the second phase structure may be a martensite structure, a bainite structure, or a high strength low alloy (HSLA) steel structure (e.g., a dual phase (DP) steel structure, a transformation induced plasticity (TRIP) steel structure, or a twinning induced plasticity (TWIP) steel structure).

A mixed portion may be disposed between the first portion and the central portion and/or between the second portion and the central portion. The mixed portion may have both the first phase and the second phase. The volume fraction of the first phase of the mixed portion may gradually increase as a distance from each of the first and second surfaces 10 a and 10 b decreases, and the volume fraction of the second phase of the mixed portion may gradually decrease as a distance from each of the first and second surfaces 10 a and 10 b decreases. The volume fraction of the second phase of the mixed portion may gradually increase as a distance from the central portion decreases, and the volume fraction of the first phase of the mixed portion may gradually decrease as a distance from the central portion decreases.

The first and second portions having only the first phase, the central portion having only the second phase, and the mixed portion having both the first and second phases will be described in more detail with reference to FIG. 6.

FIG. 6 is a cross-sectional view taken along the line A-A′ of FIG. 3 to illustrate a formed product according to some embodiments of the inventive concepts.

Referring to FIG. 6, the formed product 10 may include a first portion 11, a first mixed portion 12, a central portion 13, a second mixed portion 14, and a second portion 15 which are sequentially stacked. The first portion 11, the first mixed portion 12, the central portion 13, the second mixed portion 14, and the second portion 15 may be in one body.

The first portion 11 and the second portion 15 may have only a first phase, and the central portion 13 may have only a second phase different from the first phase. The first mixed portion 12 and the second mixed portion 14 may have both the first phase and the second phase.

A structure of the first phase may have a higher elongation than a structure of the second phase. The second phase structure may have a higher strength than the first phase structure. For example, the first phase structure may be a ferrite structure, and the second phase structure may be a martensite structure, a bainite structure, or a high strength low alloy (HSLA) steel structure (e.g., a dual phase (DP) steel structure, a transformation induced plasticity (TRIP) steel structure, or a twinning induced plasticity (TWIP) steel structure).

A volume fraction of the first phase in the first mixed portion 12 may gradually increase as a distance from the first portion 11 decreases, and a volume fraction of the second phase in the first mixed portion 12 may gradually increase as a distance from the central portion 13 decreases. Thus, the volume fraction of the first phase may be higher than the volume fraction of the second phase in a region of the first mixed portion 12, which is adjacent to the first portion 11. The volume fraction of the second phase may be higher than the volume fraction of the first phase in another region of the first mixed portion 12, which is adjacent to the central portion 13.

A volume fraction of the first phase in the second mixed portion 14 may gradually increase as a distance from the second portion 15 decreases, and a volume fraction of the second phase in the second mixed portion 14 may gradually increase as a distance from the central portion 13 decreases. Thus, the volume fraction of the first phase may be higher than the volume fraction of the second phase in a region of the second mixed portion 14, which is adjacent to the second portion 15. The volume fraction of the second phase may be higher than the volume fraction of the first phase in another region of the second mixed portion 14, which is adjacent to the central portion 13.

Equipment for forming a steel sheet according to some embodiments of the inventive concepts will be described hereinafter with reference to FIGS. 7 to 10.

FIG. 7 is a schematic view illustrating equipment for forming a steel sheet, according to some embodiments of the inventive concepts, and FIG. 8 is a perspective view illustrating a surface cooling part included in equipment for forming a steel sheet, according to some embodiments of the inventive concepts.

Referring to FIGS. 7 and 8, the equipment for manufacturing the steel sheet may include a cutting part 110, a heat supply part 120, a surface cooling part 130, and a forming/cooling part 140.

The cutting part 110 may cut a steel sheet S of a coil to a predetermined size. The steel sheet S may be a hot-rolled steel sheet or a cold-rolled steel sheet, as described with reference to FIG. 1.

The heat supply part 120 may thermally treat the steel sheet S cut by the cutting part 110. The thermally treated steel sheet S may be phase-transformed as described with reference to FIG. 1. In some embodiments, the thermally treated steel sheet S may have an austenite structure.

The surface cooling part 130 may selectively cool a surface portion of the thermally treated steel sheet S. The surface cooling part 130 may include a first surface cooling part 130 a cooling a first surface of the steel sheet S and a second surface cooling part 130 b cooling a second surface of the steel sheet S. The second surface may be opposite to the first surface. The first surface cooling part 130 a and the second surface cooling part 130 b may be spaced apart from each other with the steel sheet S interposed therebetween.

Each of the first and second surface cooling parts 130 a and 130 b may include a temperature-reducing agent supply line 132 and a temperature-reducing agent supply nozzle 134. The steel sheet S may be transferred in a first direction D, and the temperature-reducing agent supply line 132 may extend in a second direction perpendicular to the first direction D. The temperature-reducing agent supply nozzle 134 may be installed on the temperature-reducing agent supply line 132 and may supply a temperature-reducing agent supplied from the temperature-reducing agent supply line 132 to the first surface or the second surface of the steel sheet S. In some embodiments, the temperature-reducing agent supply nozzle 134 may be provided in plurality, and the temperature-reducing agent supply nozzles 134 may be spaced apart from each other in the second direction.

The temperature-reducing agent provided to the first and second surfaces of the steel sheet S through the temperature-reducing agent supply nozzles 134 may be, for example, compressed air or liquid. When the temperature-reducing agent is the liquid temperature-reducing agent, the temperature-reducing agent may be jetted from the temperature-reducing agent supply nozzles 134 toward the first surface and the second surface.

The temperature-reducing agent supply nozzles 134 may supply the temperature-reducing agent to the first surface and the second surface of the steel sheet S in a direction opposite to the first direction D in which the steel sheet S is transferred. Thus, the temperature-reducing agent supplied from the temperature-reducing agent supply nozzles 134 may become in contact with the first and second surfaces of the steel sheet S and may be then moved in the direction opposite to the first direction D. As a result, the first and second surfaces of the steel sheet S may be cooled in a moment, and the temperature-reducing agent may not be supplied again to the surface of the steel sheet S previously supplied with the temperature-reducing agent. Thus, a central portion of the steel sheet S may not be cooled but a first portion of the steel sheet S adjacent to the first surface and a second portion of the steel sheet S adjacent to the second surface may be selectively cooled.

The first portion and the second portion of the steel sheet S may be selectively cooled by the first and second surface cooling parts 130 a and 130 b, thereby performing the first phase transformation process described with reference to FIG. 1. In more detail, the first and second surfaces of the steel sheet S may be cooled by the first and second surface cooling parts 130 a and 130 b, and thus the first and second portions adjacent to the first and second surfaces of the steel sheet S may be phase-transformed. While the steel sheet S is cooled by the first and second surface cooling parts 130 a and 130 b, the central portion of the steel sheet S may not be cooled such that a phase of a structure of the central portion of the steel sheet S may not be transformed.

The forming/cooling part 140 may form the steel sheet S of which structures of surface portions (i.e., the first and second portions) are phase-transformed. In addition, the forming/cooling part 140 may cool the central portion of the formed steel sheet S to manufacture a formed product 10. In some embodiments, the forming/cooling part 140 may water-cool the formed steel sheet S by supplying a great amount of water to the formed steel sheet S. Thus, the second phase transformation process described with reference to FIG. 1 may be performed. In more detail, the central portion of the formed steel sheet S may be phase-transformed by the water supplied in the forming/cooling part 140 but the phases of the structures of the surface portions adjacent to the first and second surfaces may not be transformed.

In the equipment for forming the steel sheet described with reference to FIG. 7, the steel sheet S may be formed after the first phase transformation process is performed on the steel sheet S, thermally treated by the heat supply part 120, by the surface cooling part 130. Alternatively, the first phase transformation process and the second phase transformation process may be sequentially performed after the thermally treated steel sheet S is formed, as described with reference to FIG. 2. This will be described with reference to FIG. 9.

FIG. 9 is a schematic view illustrating equipment for forming a steel sheet, according to other embodiments of the inventive concepts.

Referring to FIG. 9, the cutting part 110, the heat supply part 120, and the forming/cooling part 140 described with reference to FIG. 7 may be provided but the surface cooling part 130 may be omitted. In the equipment for forming the steel sheet according to the present embodiment, the steel sheet S thermally treated by the heat supply part 120 may be formed in the forming/cooling part 140, and then, the first phase transformation process and the second phase transformation process may be sequentially performed on the formed steel sheet S in the forming/cooling part 140, as described with reference to FIG. 2. In other words, the steel sheet S may be thermally treated by the heat supply part 120 such that a structure of the steel sheet S may be phase-transformed (e.g., to an austenite structure), and then, the steel sheet S may be formed in the forming/cooling part 140. Next, a temperature-reducing agent may be supplied to the formed steel sheet S to phase-transform the structure of the surface portion of the formed steel sheet S (e.g., to a ferrite structure), and then, water may be supplied to the formed steel sheet S to phase-transform the structure of the central portion of the formed steel sheet S (e.g., to a martensite or bainite structure).

Unlike the equipment described with reference to FIG. 9, the first phase transformation process and the second phase transformation process may not be performed in the forming/cooling part 140 but may be sequentially performed on the formed steel sheet S in an apparatus different from the forming/cooling part 140. This will be described with reference to FIG. 10.

FIG. 10 is a schematic view illustrating equipment for forming a steel sheet, according to still other embodiments of the inventive concepts.

Referring to FIG. 10, the cutting part 110, the heat supply part 120, the surface cooling part 130, and the forming/cooling part 140 described with reference to FIG. 7 may be provided. Unlike the equipment described with reference to FIG. 7, the surface cooling part 130 may not perform the first phase transformation process on the steel sheet S thermally treated by the heat supply part 120 but may perform the first phase transformation process on the steel sheet S formed by the forming/cooling part 140. As described with reference to FIG. 2, the second phase transformation process may be performed on the steel sheet S on which the first phase transformation process was performed. Thus, the formed product 10 may include the surface portion having the structure of a relatively high elongation and the central portion having the structure of a relatively high strength.

FIG. 11 is a scanning electron microscopy (SEM) image illustrating micro-structures of a steel sheet according to some embodiments of the inventive concepts.

Referring to FIG. 11, a steel sheet was prepared to have carbon (C) of 0.15 wt %, manganese (Mn) of 1.2 wt %, silicon (Si) of 0.3 wt %, niobium (Nb) of 0.03 wt %, boron (B) of 0.02 wt %, a residual iron (Fe), and other inevitable impurities. The steel sheet has a thickness of 40 mm and an austenite structure. Compressed air was provided to the steel sheet thermally treated to perform the first phase transformation process described with reference to FIG. 1. Thus, a surface portion of the steel sheet was phase-transformed from the austenite structure to a ferrite structure by the first phase transformation process. Thereafter, the steel sheet was water-cooled to perform the second phase transformation process described with reference to FIG. 1. Thus, a central portion of the steel sheet was phase-transformed from the austenite structure to a martensite structure by the second phase transformation process.

As illustrated in FIG. 10, the surface portion of the steel sheet supplied with the compressed air had the ferrite structure having a relatively high elongation, and the central portion of the steel sheet had the martensite structure having a relative high strength.

In addition, a mixed portion between the surface portion and the central portion had both the ferrite structure and the martensite structure. In particular, a volume fraction of the ferrite structure of the mixed portion became progressively greater toward the surface portion, and a volume fraction of the martensite structure of the mixed portion became progressively greater toward the central portion.

According to some embodiments of the inventive concepts, the structures of the surface portion and the central portion of the steel sheet may be phase-transformed before or after forming the thermally treated steel sheet, and thus the surface portion may have the relatively high elongation and the central portion may have the relatively high strength. As a result, embodiments of the inventive concepts may provide the method of forming the steel sheet having the high elongation, the high workability, and the high strength, the equipment for forming the same, and the formed product.

While the inventive concepts have been described with reference to exemplary embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirits and scopes of the inventive concepts. Therefore, it should be understood that the above embodiments are not limiting, but illustrative. Thus, the scopes of the inventive concepts are to be determined by the broadest permissible interpretation of the following claims and their equivalents, and shall not be restricted or limited by the foregoing description. 

What is claimed is:
 1. A method of forming a steel sheet, the method comprising: preparing a steel sheet including a surface portion and a central portion; thermally treating the steel sheet; performing a first phase transformation process cooling a surface of the thermally treated steel sheet to phase-transform a structure of the surface portion of the thermally treated steel sheet; forming the steel sheet after the performing of the first phase transformation process; and performing a second phase transformation process cooling the formed steel sheet to phase-transform a structure of the central portion of the formed steel sheet.
 2. The method of claim 1, wherein a phase of the structure of the central portion of the steel sheet is maintained during the performing of the first phase transformation process.
 3. The method of claim 2, wherein a phase of the structure of the surface portion of the steel sheet is maintained during the forming of the steel sheet and the performing of the second phase transformation process.
 4. The method of claim 1, wherein the thermally treated steel sheet has an austenite structure, wherein the structure of the surface portion of the thermally treated steel sheet is phase-transformed to a ferrite structure by the first phase transformation process, wherein the austenite structure of the central portion of the thermally treated steel sheet is maintained during the performing of the first phase transformation process, wherein the central portion of the formed steel sheet has a structure having a higher strength than the ferrite structure by the second phase transformation process, and wherein the ferrite structure of the surface portion of the formed steel sheet is maintained during the performing of the second phase transformation process.
 5. The method of claim 1, wherein an elongation of the surface portion phase-transformed by the first phase transformation process is higher than an elongation of the central portion phase-transformed by the second phase transformation process, and wherein a strength of the central portion phase-transformed by the second phase transformation process is higher than a strength of the surface portion phase-transformed by the first phase transformation process.
 6. The method of claim 1, wherein the performing of the first phase transformation process comprises: supplying compressed air to the surface portion of the steel sheet.
 7. The method of claim 1, wherein the performing of the first phase transformation process comprises: jetting a liquid temperature-reducing agent to the surface portion of the steel sheet.
 8. The method of claim 1, wherein a step of forming the steel sheet and the second phase transformation process are performed in the same apparatus.
 9. A formed product comprising: a surface portion adjacent to an exterior surface; and a central portion surrounded by the surface portion, wherein a first phase has a maximum volume fraction in the surface portion, wherein a second phase has a minimum volume fraction in the surface portion, wherein the second phase has a maximum volume fraction in the central portion, and wherein the first phase has a minimum volume fraction in the central portion.
 10. The formed product of claim 9, wherein a structure of the first phase is a ferrite structure, and wherein a structure of the second phase has a higher strength than the ferrite structure.
 11. The formed product of claim 9, wherein the surface portion has only the first phase, and wherein the central portion has only the second phase.
 12. The formed product of claim 9, wherein the volume fraction of the first phase gradually decreases from the surface portion to the central portion, wherein the volume fraction of the second phase gradually increases from the surface portion to the central portion, wherein the volume fraction of the second phase gradually decreases from the central portion to the surface portion, and wherein the volume fraction of the first phase gradually increases from the central portion to the surface portion.
 13. The formed product of claim 9, wherein a structure of the first phase has a higher elongation than a structure of the second phase, and wherein the structure of the second phase has a higher strength than the structure of the first phase.
 14. The formed product of claim 9, further comprising a mixed portion between the surface portion and the central portion including the first phase and the second phase.
 15. Equipment for forming a steel sheet, the equipment comprising: a cutting part cutting a steel sheet; a heat supply part thermally treating the cut steel sheet; a surface cooling part selectively cooling a surface portion of the thermally treated steel sheet; and a forming/cooling part forming the steel sheet having the cooled surface portion and cooling a central portion of the formed steel sheet.
 16. The equipment of claim 15, wherein the surface cooling part comprises: a first surface cooling part cooling a first surface of the steel sheet; and a second surface cooling part cooling a second surface of the steel sheet opposite to the first surface.
 17. The equipment of claim 15, wherein the first surface cooling part and the second surface cooling part supply a temperature-reducing agent to the first surface and the second surface in a direction opposite to a direction in which the steel sheet is transferred.
 18. The equipment of claim 17, wherein temperature-reducing agent supply nozzles supplying the temperature-reducing agent are disposed obliquely to the first and second surfaces of the steel sheet.
 19. The equipment of claim 15, wherein the surface cooling part supplies compressed air to selectively cool a surface portion of the steel sheet.
 20. The equipment of claim 15, wherein the surface cooling part selectively cools the surface portion of the thermally treated steel sheet except the central portion to phase-transform a structure of the surface portion, and wherein the forming/cooling part cools the formed steel sheet to phase-transform a structure of the central portion except the surface portion. 